1

Lists

2

Introduction

Data: A finite sequence of data items. Operations: Construction: Create an empty list Empty: Check if list is empty Insert: Add an item at any position in the list Delete: Remove an item from the list at any position in the list Traverse: access and process elements

in order of occurrence

3

Possible Implementations

• Array– capacity determined at compile-time– capacity determined at run-time– good choice if

• capacity known before list is constructed• insertions/deletions mainly at the end

• Linked List (dynamic structure)– capacity grows and shrinks as size changes– insertions/deletions do not require shifting – access to an item by index is not efficient

Structural Concept

• Each element is a node• Space is dynamically allocated (and

returned) one node at a time• Items are not contiguous in memory

linked list (dynamic)

• node contains >=1 data item and pointer to next node

• The last node's "next" pointer is "empty"• A separate pointer accesses first node

first

data next

Accessing nodes

• no direct access to individual nodes• nodes only accessed via pointers• access types

– a list is a sequential access data structure• Because you cannot get directly to an item

– an array is a direct access data structure• Because a subscript gets you directly to an item

7

Implementation

typedef Complx QueueElement;

struct QueueNode

{QueueNode * next;

QueueElement XX;

};

void enqueue (QueueNode * , Complx);

QueueNode * myFront; // Declare anchor

myFront= (QueueNode *) malloc (sizeof(QueueNode));

// above stmt creates list node 0, sets myFront

Complx X;

enqueue (myFront, X); // put X in the list

8

Efficiency

• How do you insert?• How do you extract?• What about access in the "middle"?• Are some ways easier?

struct Node

{

int X;

Node * next;

};

Inserting at position 0

// allocate space for a node & store item in itNode * aNode = new Node (item);

aNode -> next = first; // new node -> old head of list

first = aNode; //anchor -> new head of list

first

addedNode

10

Inserting between the ends

• Locate position for insertion• Save "next"• Create new item• Change next -> new item• Store saved "next" in new item's "next"

first

addedNodetemp

11

Removing a node

• Must be careful• Save the ptr to the node BEFORE the node

to be removed.• May have to "peek" at next item to decide if

you’re there yet• Save the "next" ptr of the node to remove• Put the saved "next" pointer into the "next"

of the previous node• Free old node (malloc/free or new/delete)

12

DANGER!!!

• When removing a node– must save a ptr to it if re-use is possible– must delete everything the node points to

• free won't "chase down" additional storage

firsttemp

not freed

13

Traversal

curr_ptr = first;

while (curr_ptr != NULL)

{compare data for correct node

curr_ptr=curr_ptr -> next; //advance

}

first

ptr to "current" node

14

Two-way lists

struct Node

{ int X;

Node * next; // points forward in list

Node * prev; // points backward in list

};

• Insert & Delete functions more complex• Must have (or get) pointers to both sides

first

addedNodetemp

15

Two-way lists-2

C=// the current node (maybe found with a search)

N=C->next; // save the tail_pointer

P=(Node*) malloc (Node); // get new node

P->next=N; // new node points to old tail of list (1)

P->prev=C; // cutoff point points to new node (2)

C->next=P; // old head points to new node (3)

N->prev=P; // old tail points to new node (4)

first

addedNode

CN

P 1

23

4